Downhole mechanical energy absorbers can be used to protect equipment in a wellbore from dynamic loads that can arise from several sources. These sources include impacts that occur during tool run-in or that occur when tools are dropped into the wellbore. The source of the mechanical load can also be an explosive blast such as the detonations that occur during perforation operations.
The dynamic loads may vary greatly in scale of magnitude and in duration. For example, a blast load may have peaks of less than a millisecond and may induce forces of hundreds of thousands of pounds. A tool drop impact may have a slower onset but may perhaps have an even larger total kinetic energy. On the opposite end of the load spectrum, a longer duration loading event may have a gradual onset, such as when weight is set down on a tool string.
The function of a mechanical energy absorber is to absorb mechanical energy and convert kinetic energy into heat and into a controlled material deformation. In doing so, the loads that affect the tool string can be limited in magnitude. In this manner a mechanical energy absorber is both an energy absorber and a load limiter. In some downhole applications, limiting peak loads in the tool string is the primary objective of the absorber. This may also include buckling prevention. In other downhole applications, the objective may be to protect sensitive equipment within the tools, such as electronics, from high acceleration levels or shock loads. In all cases, the wellbore diameter is a major design constraint.
Many different types of mechanical energy absorbers have been developed over the past half century. Mechanical energy absorbers (also referred to as shock absorbers) have been used not only in downhole applications but also in aircraft and ground vehicles to provide protection for passengers and sensitive cargo. Many of the designs for vehicles, particularly in the aerospace industry, focus on optimizing a specific energy absorbed, where weight is a penalty. The objective of these designs is to reduce acceleration magnitude and duration to improve survivability.
The ideal linear-motion mechanical energy absorbers provide a constant load during their stroke. This load may be set to a maximum allowable level without causing damage to the systems being protected. For the ideal constant load design, the mechanical energy absorber exhibits a near zero spring-rate during its stroke. Low noise or ripple during the stroke also reduces shock loads transferred across the device.
In order to operate over the full range of downhole loading conditions, a device must be able to handle very rapid onset of force as well as handle high levels of energy absorbed. The inelastic deformation of metals has been demonstrated to be one of the best performing and most versatile and reliable means of absorbing significant mechanical energy in a continuous and uniform fashion. Because wellbore conditions and tool string geometry vary greatly, it is imperative to have a design that can be readily adapted to meet the specific requirements of each job.
Some of the simplest energy absorbers rely primarily on the elastic compression of springs or elastomeric elements. The problem with these designs is that the force is not constant but rather increases during the stroke, and the energy is only stored temporarily and thus is not truly absorbed. This results in a rebound with similar potentially damaging effects as the original shock. Performance can also change significantly with temperature.
Other designs have utilized discretized energy absorbing elements that sequentially engage to absorb energy. Each element absorbs energy only over a small portion of the total stroke of the device. The drawback of such concepts is that the load level is still not constant and will have a significant ripple or noise level.
Other designs rely on frictional forces to dissipate energy as heat. The drawback of these concepts is also a non-constant load level and a build up of heat that can lead to damage and performance degradation, particularly in a long-stroke application.
Fluid-based concepts for dissipating energy typically force a fluid through an orifice similar to automotive shocks used between the wheel or axle and the vehicle frame. These devices depend upon viscous damping and fluid shear and are highly rate-dependent and are not feasible for high rate impact or ballistic shocks. At high rates, the fluid flow cannot respond to the rapid load onset causing the forces to escalate with minimal stroke.
Applications involving vibration isolation, vibration damping, or small-amplitude wave attenuation are not directly relevant. Designs for such applications cannot absorb energy on the scale required for the intended impact and shock events described here. Instead, these designs typically focus on protecting electronics from long duration vibratory loads such as those generated while drilling.
Prior art downhole mechanical energy absorbers include the following:
U.S. Pat. No. 3,653,468 is one of the earlier downhole shock absorbers and utilizes sequential shearing of metal disks or washers to absorb energy.
U.S. Pat. No. 4,679,669 cuts chips from a mandrel using shearing cutters fixed to the housing, similar to machine tools such as on a lathe.
U.S. Pat. Nos. 5,131,470 and 5,366,013, as well as U.S. Pat. App. 2006/0118297, describe honeycomb crushing along with a damping coil for shock absorption.
U.S. Pat. No. 5,188,191 utilizes alternating metal and rubber layers to provide an impedance mismatch for reducing shock transmitted.
U.S. Pat. Nos. 6,109,355 and 6,454,012 describe a frictional interference fit with elastic deformation for energy absorption. The patents describe a uniform deformation of the tool housing with hoop stress being the primary design metric. The deformation may also be inelastic for one-time use applications.
U.S. Pat. No. 6,708,761 employs the sequentially shearing of radially-oriented metal elements, or shear pins, to absorb energy during a linear stroke.
U.S. Patent Application Publication No. 2003/0150646 uses a porous material to absorb shock loads.
U.S. Patent Application Publication No. 2004/0140090 transfers shock energy to a spring-mass system.
A more extensive history of continuously deforming or rupturing metal for energy absorption can be found outside of the oil and gas producing industry. These devices are intended for absorbing energy from the relative motion between vehicles, between vehicles and the environment, or occupants and the vehicles themselves.
U.S. Pat. No. 3,143,321 describes the continuous rupturing of a tube forced onto a die. Similar concepts were also described in NASA technical report NASA TN D-5730.
Another NASA report, NASA TN D-4941, describes a tube cutter design that cuts a tube into longitudinal strips to absorb energy for a landing gear strut. A similar approach is used in U.S. Pat. No. 5,547,148.
U.S. Pat. No. 3,394,612 utilizes interference between telescoping tubes such that the outer tube is deformed by embossments on the inner tube for a vehicle steering column energy absorber.
U.S. Pat. No. 3,779,591 uses fixed cutters to shear away material from a moving mandrel.
Similarly, U.S. Pat. No. 4,346,795 uses a cutting ring to shear away the entire circumference of the mandrel surface.
U.S. Pat. No. 4,575,026 describes a continuous plastic deformation of metal to decelerate a vehicle on a track.
U.S. Pat. No. 5,351,791 discloses a tube pushed through a reducing die and a crushing element.
U.S. Pat. No. 6,135,252 relies on metal extrusion for energy absorption.
U.S. Pat. Nos. 6,308,809 and 6,457,570 introduce stress concentrators to control the rupture of a tube.
U.S. Pat. No. 7,147,088 describes the controlled collapse of thin-walled tubes in a multi-stage design.
U.S. Pat. No. 6,371,541 shear cuts material from a metal structure with guides to control the direction of the progression.
U.S. Pat. No. 6,394,241 applies a combination of shearing and bending to absorb energy for crashworthy seats. The inventor, Desjardins, also published a history and summary of energy absorbers used for crashworthy seats in an American Helicopter Society (AHS) presentation and paper (AHS 59th Annual Forum, May 2003).